Valmiki Ramayana Yuddha Kanda Sarga 24 Shloka 34

राक्षसां Rakshasas, बलौघस्य for Bears, वानरेन्द्रबलस्यच for army of Vanara king's, एतयोः for both of them, देवदानवयोरिव like for gods and demons, सन्धि: alliance, नविद्यते not possible.

Alliance is not possible between Rakshasas and army of Bears and Vanaras, just as alliance is not possible between gods and demons.